Again, we present another beloved classroom demonstration in materials science. This one is a non-intuitive display of surface tension, residual stress, interior tension, potential energy and tempered glass.

To create a Prince Rupert drop, molten glass is dropped into cold water. The glass rapidly forms into teardrop shape with a extended, fine tail. The material in the exterior of the drop cools and hardens nearly immediate while the interior material cools slowly. As the interior material cools, it contracts and sets up powerful compressive stresses on the surface.

The residual stress within the drop gives rise to unique properties that every demonstrator loves: The drop can be hammered on the fat end without breaking, but disintegrates explosively if the tail end is even slightly damaged. This illustrates the release of the potential energy contained within the drop’s amorphous atomic structure.

The process of the “explosion” has been studied closely and shows that fractures move from the tail through the material at very high speed. Purdue University’s Srinivasan Chandrasekar used extremely high speed video to record how the “crack front” propagates in a disintegrating drop at up to 4,200 miles per hour.

The drops were supposedly discovered around the 1640s by Prince Rupert of the Rhine (1619–1682). The story is told that Rupert would use the drops as a practical joke in his court. He would give a drop to someone in his audience and then surreptitiously break the tail causing a small and surprising explosion.

We have actually combined two videos. The second part is slightly repititious but provides both a slo-mo view of the shattering process and a lovely image of namesake Rupert.

5 Responses to Video of the week – Prince Rupert’s Drops

Thanks to Casey Crandle for catching the crystalline phrase. I would like a copy of this video as well. Also, is there any possibility of seeing Dr. Chandrasekar’s slow motion video that show the crack front propogating?

Rupert drops are great fun. I especially love the polarized light image. Its simple to look at and see the basics of whats going on, really ties the fun of exploding little droplets with the chance to learn and gain understanding about things. Thanks!

I loved this Rupert drops experience. As a ceramist and also glass science background, I must take issue to the comment: “This illustrates the release of the potential energy contained within the drop’s crystalline atomic structure.” Glass is amorphous not crystalline! That is the entire reason for this unique material and its response. When we can get amorphous-glass like metals that do exist now, you will never get it to do treat or respond the same. Viva SiO2

I saw Prince Rupert drops when I worked as a lab assistant at Corning Glass in 1956. There was a technician who melted glass in a Platinum crucible to run viscosity tests on the molten glass. I think that the viscosity test had been developed by Howard Lilly who happened to be my supervisor. I was told that in the glass plant some of the old timers would form small Prince Rupert drops and then have a new employee hold the drop and they would break the stem. I understand that it felt like the palm of the hand was hit with a baseball bat.